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Will be used to implement a scrollview that does not extend beyond the
boundaries of a widget with rounded corners. Necessary because
scrollviews paint on a layer so they will paint over the parent view's
bounds. Also similar code creating rounded backgrounds happens in a
bunch of places in the codebase and could be simplified to use a single
common background factory method (example: bubble background code).

Change-Id: Ie885ca9f71f2722680e6fb709fe3091e71ad88fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524910
Commit-Queue: Sajad Maysam <sajadm@google.com>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644468}